For the purposes of the present discussion, the present invention will be discussed in terms of a “white” emitting light-emitting diode (LED); however, the methods taught in the present invention can be applied to wide range of LEDs. A “white emitting LED” is an LED that emits light that is perceived by a human observer to be “white”. Such sources can be constructed by making an LED that emits a combination of blue and yellow light in the proper ratio of intensities. High intensity blue-emitting LEDs are known to the art. Yellow light can be generated from the blue light by converting some of the blue photons via an appropriate phosphor. In one design, a transparent layer containing dispersed particles of the phosphor covers an LED chip. The phosphor particles are dispersed in a potting material that surrounds the light-emitting surfaces of the blue LED. To obtain a white emitting LED, the thickness and uniformity of the dispersed phosphor particles must be tightly controlled.
In one class of prior art LEDs, the phosphor layer is fabricated by a molding process that utilizes a liquid mold compound that has the phosphor particles dispersed therein. The liquid mold compound is applied to a die having an LED thereon. The mold compound is then cured in place to provide the layer of phosphor particles. In one design, the LED is mounted on a heat sink in a well in a printed circuit board base. The well has reflective sides that form a reflective “cup” having the LED chip at the bottom thereof. The phosphor is mixed with a liquid casting epoxy and injected into the cup. The mixture is then heat-cured for 2 hours.
Unfortunately, this manufacturing system has a poor yield due to uneven phosphor dispersion in the reflecting cup. The density of the phosphor particles is greater than that of the liquid casting epoxy, and hence, the particles tend to settle toward the bottom of the reflector cup during the curing period. As a result, the amount of phosphor over the chip is reduced, which, in turn, lowers the ratio of yellow to blue light generated by the completed device. Such a device emits light that is bluish-white rather than white.